Two subspecies of the polypeptide chain initiation factors IF3 in E. coli exhibit high selectivity towards different messengers. IF3 alpha selects for E. coli, coliphage (e.g. MS2, R17) and early T4 RNA whereas IF3 beta selects for late T4 RNA. The discovery of factors in E. coli specifically inhibiting translation of either MS2, or late T4 RNA, suggests translational control by these proteins. Two factors i alpha and i beta were isolated, the former in homogenous form, the latter will be further purified. Their mode of action, whether complexing with IF3, free or ribosome bound, or with mRNA will be studied. For this purpose, attempts will be made to introduce radioactive label in these factors. Recently a new inhibitor of polypeptide chain initiation was isolated from E.coli. This inhibition is reversible. This might function in translational control. Further studies will involve purification and characterization of this protein and its mode of action and the mechanism of release of its inhibitory activity. Studies on the nature and mode of action of eukaryotic initiation factors will be continued. Previous work led to the isolation of the eukaryotic counterpart (EIF1) of the bacterial initiation factor IF2 from embryos of the brine shrimp Artemia salina. Artemia embryos do not seem to contain the equivalent of bacterial IF3 but such a factor was isolated by others from reticulocytes. A cell-free system, consisting in part of elements from A. salina embryos (initiation factor EIF1, ribosomal subunits) will be used for isolation and purification of IF3 from eukaryotic sources. This is feasible because, ribosomal subunits and EIF1 from A. salina embryos, rat liver, reticulocytes, and other eukaryotic sources are interchangeable. Recent work on the properties of highly purified elongation factors, EF1 and EF2 from Artemia embryos, specially on the conformational control of the ribosomal binding of these factors will be continued.